[fonts-4] Define @font-palette-values with Syntax grammar.

Author: tabatkins

css-fonts-4/Overview.bs

@@ -2724,21 +2724,15 @@ with colors described by the web author.
 
 The ''@font-palette-values'' rule consists of the ''@font-palette-values'' at-keyword
 followed by a block of descriptor declarations.
-In terms of the grammar, this specification defines the following productions:
+It has the following syntax:
 
-<pre>
-<dfn>font_palette_values_rule</dfn>
-  : <a>FONT_PALETTE_VALUES_SYM</a> <a href="https://www.w3.org/TR/CSS21/grammar.html#scanner"><a>S</a></a>* '{' <a href="https://www.w3.org/TR/CSS21/grammar.html#scanner"><a>S</a></a>* <a>descriptor_declaration</a>? [ ';' <a href="https://www.w3.org/TR/CSS21/grammar.html#scanner"><a>S</a></a>* <a>descriptor_declaration</a>? ]* '}' <a href="https://www.w3.org/TR/CSS21/grammar.html#scanner"><a>S</a></a>*
-  ;
-
-<dfn>descriptor_declaration2</dfn>
-  : <a href="https://www.w3.org/TR/CSS21/grammar.html#grammar"><a>property</a></a> ':' <a href="https://www.w3.org/TR/CSS21/grammar.html#scanner"><a>S</a></a>* <a href="https://www.w3.org/TR/CSS21/grammar.html#grammar"><a>expr</a></a>
-  ;
+<pre class=prod>
+	@font-palette-values {
+		<<declaration-list>>
+	}
 </pre>
 
-The following new token is introduced:
-
-<pre>@{F}{O}{N}{T}{-}{P}{A}{L}{E}{T}{T}{E}{-}{V}{A}{L}{U}{E}{S} {return <dfn>FONT_PALETTE_VALUES_SYM</dfn>;}</pre>
+The ''@font-palette-values'' rule accepts the descriptors defined in this specification.
 
 <div class="example">
 	Modify Handover Sans's color palette
@@ -2755,7 +2749,7 @@ The following new token is introduced:
 
 The presence of a particular descriptor within in ''@font-palette-values'' has semantic meaning;
 missing descriptors are not filled in with default values.
-These descriptors apply solely within the context of the @font-palette-values rule in which they are defined,
+These descriptors apply solely within the context of the ''@font-palette-values'' rule in which they are defined,
 and do not apply to document language elements.
 When a given descriptor occurs multiple times in a given ''@font-palette-values'' rule,
 only the last descriptor declaration is used
@@ -2776,9 +2770,9 @@ This at-rule follows the forward-compatible parsing rules of CSS.
 Like properties in a declaration block,
 declarations of any descriptors that are not supported by the user agent
 must be ignored.
-@font-palette-values rules require a font-family descriptor;
+''@font-palette-values'' rules require a '@font-palette-values/font-family' descriptor;
 if it is missing,
-the @font-palette-values rule is invalid and must be ignored entirely.
+the ''@font-palette-values'' rule is invalid and must be ignored entirely.
 
 In cases where user agents have limited platform resources,
 do not implement support for color fonts,